Capacitive pressure sensor

ABSTRACT

A high-accuracy high-stability capacitor type pressure sensor which eliminates a parasitic capacitance between a reference capacitor and a semiconductor substrate. A capacitor type pressure sensor comprising, on a semiconductor substrate  10 , an active capacitor  100  whose capacitance varies as the surrounding pressure varies, a reference capacitor  200  whose capacitance will not vary substantially as the surrounding pressure varies, and a circuit which is electrically connected to both said active and reference capacitors  100  and  200,  detects the difference or ratio thereof, and uses the potential of a semiconductor substrate, wherein an electrode  30   a  of said reference capacitor is formed on the semiconductor substrate  10  with a dielectric  20  therebetween.

FIELD OF THE INVENTION

The present invention relates to a pressure sensor for detecting apressure of fluid, particularly to a pressure sensor used to control acar engine and the like which is made using a semiconductor fineprocessing technology.

PRIOR ART

For example, Japanese Non-Examined Patent Publication No.7-7162 (1995)discloses a conventional pressure sensor.

The conventional pressure sensor is a capacitor type pressure sensorformed on a semiconductor substrate, comprising a reference capacitorwhich remains constant independently of a surrounding pressure and asensing capacitance which varies as the surrounding pressure varies.Each capacitor consists of a first electrode which is a diffusion layerformed directly on a semiconductor substrate and a second electrode of aflexible diaphragm which is formed opposite to said first electrode witha cavity between said first and second electrodes and contains aconductive area made of single-crystal silicone. The cavity is sealed tokeep a preset pressure. Both reference and sensing capacities use adiffusion layer formed on the semiconductor substrate as the firstelectrode. The pressure sensor disclosed by Japanese Non-Examined PatentPublication No.7-7162 (1995) is characterized in that the flexiblediaphragm moves as the surrounding pressure varies and thus thecapacitance between the first and second electrodes varies.

DISCLOSURE OF THE INVENTION

For higher accuracy, a well-known conventional capacitor type pressuresensor employs a technology comprising the steps of

using a reference electrode which has a capacitance almost equal to thatof an active capacitor and will not vary substantially as means tocancel a characteristic change of the active capacitor due to externaldisturbing factors such as noises and uneven processing which is notrelated to the pressure change and

calculating a capacitance difference between the reference capacitor andthe active capacitor or a ratio thereof by a detecting circuit.

For higher accuracy of a capacitor type pressure sensor comprising areference capacitor, an active capacitor and a detecting circuit on asemiconductor substrate, there may arise a problem that an unwantedparasitic capacitance (or a junction capacitance) which may vary due tothe concentration of impurities in the substrate or a potentialdifference between the substrate and the reference capacitor electrodegenerates between the reference capacitor electrode and thesemiconductor substrate. Therefore, when the semiconductor substrate isgrounded or connected to a detecting circuit which is used as a powersupply, the parasitic capacitance between the electrode and thesemiconductor substrate varies greatly relative to a preset capacitanceformed between reference capacitor electrodes and the SN ratio of thereference capacitor including the parasitic capacitance relative to thechange of the active capacitor increases and varies. Consequently theaccuracy of measurement of fluid pressure will go down.

For a capacitor type pressure sensor comprising, on a semiconductorsubstrate, an active capacitor which varies as the surrounding pressurevaries, a reference capacitor which does not vary substantially as thesurrounding pressure varies, and a circuit which is electricallyconnected to said active capacitor and to said reference capacitor,detects a capacity difference between said capacitors or a rate thereof,and works using the potential of the semiconductor substrate, theaforesaid problem can be solved by forming a reference capacitorelectrode on the semiconductor substrate through a dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a capacitor type pressure sensorwhich is a first embodiment of the present invention.

FIG. 2 is a plan view of a capacitor type pressure sensor which is afirst embodiment of the present invention.

FIG. 3 is a cross-sectional view of a capacitor type pressure sensorwhich is a second embodiment of the present invention.

FIG. 4 is a cross-sectional view of a capacitor type pressure sensorwhich is a third embodiment of the present invention.

FIG. 5 is a plan view of a capacitor type pressure sensor which is athird embodiment of the present invention.

FIG. 6 is a plan view of a capacitor type pressure sensor which is afourth embodiment of the present invention.

FIG. 7 is a cross-sectional view of a capacitor type pressure sensorwhich is a fourth embodiment of the present invention.

FIG. 8 is a block diagram of a pressure detecting circuit of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross-sectional view of a capacitor type pressure sensorwhich is a first embodiment of the present invention. FIG. 2 shows itsplan view.

This embodiment shows a pressure detecting IC 400 of a semiconductorcapacitor type pressure sensor comprising an active capacitor 100, areference capacitor 200 and a detecting circuit 300 which are formed ona semiconductor substrate 10 with an oxide dielectric 20 between thesubstrate and these means.

The semiconductor substrate 10 is a general-purpose single-crystalsilicone substrate. For the use of a C-MOS IC which has less integrationprocesses than a bipolar IC, an n- or p-type single-crystal CZ substrateof resistivity of 8 to 12 Ωcm is employed.

The oxide dielectric 20 electrically isolates the active capacitor 100and the reference capacitor 200 from the semiconductor substrate 10. Theoxide dielectric 20 is made of a thermal oxide layer, a CVD (chemicalvapor deposition) oxide layer, etc. and the specific inductivecapacitance is approximately 3 to 4. A thermal oxide layer (field oxidelayer) can be formed together with a C-MOS IC. This can reduce theprocess steps and provide cheaper pressure sensors.

The active capacitor 100 comprises an active capacitor stationaryelectrode 30 b, a barrier dielectric layer 40, a cavity 110, and adiaphragm structure 120. The cavity 110 is hermetically sealed to keepthe almost vacuum status by a sealing dielectric 50. With this, thediaphragm structure 120 moves as the surrounding pressure varies. Thediaphragm structure 120 comprises a diaphragm electrode 120 a which isopposite to the stationary electrode 30 b and a stationary scaffold 120b. The diaphragm electrode 120 a can be obtained by making-poly-siliconeas a diaphragm structure 120 conductive by an impurity diffusion method.The stationary scaffold 120 b can be provided on the semiconductorsubstrate 10 with a barrier dielectric layer 40 between the substrate 10and the scaffold 120 b by eliminating an isolation layer in advanceusing a barrier dielectric layer 40 as an etching stop layer whencreating a cavity 110 by etching the isolation layer. This configurationenables conversion of a change of the surrounding pressure into a changeof capacitance between the active capacitor stationary electrode 30 band the diaphragm electrode 120 a. A potential between the activecapacitor stationary electrode 30 b and the diaphragm electrode 120 acan be fed to the detecting circuit 300 as explained below. Thepotential of the diaphragm electrode 120 a is led to a wiring section 60b through a wiring 30 c, a contact structure 70, and a diaphragmelectrode joint 130 (see FIG. 1). Similarly, the active capacitorstationary electrode 30 b is connected to a wiring section 60 b throughthe wiring 30 c and the contact structure 70 (see FIG. 2). The diaphragmelectrode joint 130 is an electrically-conductive structure made byeliminating part of the barrier dielectric layer 40 on the wiring 30 cwhich is formed on the oxide dielectric 20. The lower electrode 30 a,the active capacitor stationary electrode 30 b, and the wiring 30 c areconductive layers. When these layers are simultaneously processedtogether with gates of a C-MOS device such as silicide or poly-siliconelayer which underwent an impurity diffusion, process steps can bereduced and consequently cheaper pressure sensors can be provided.

The reference capacitor 200 comprises a lower electrode 30 a, a barrierdielectric layer 40, and an upper electrode 60 a. The referencecapacitor 200 is formed on the semiconductor substrate 10 with an oxidedielectric 20 therebetween. This can make the parasitic capacitancebetween the semiconductor substrate 10 and the lower electrode 30 a muchsmaller than that of conventional examples. Further the parasiticcapacitance between the semiconductor substrate 10 and the lowerelectrode 30 a does not have a voltage dependence substantially.Therefore, a high-accuracy and high-stability pressure sensor can beprovided.

FIG. 8 shows a schematic block diagram of a pressure sensor inaccordance with the present invention. This embodiment mainly comprisesa capacitance-voltage conversion section (capacitance detector) of ageneral switched capacitor type and a zero sensitivity adjuster.

Vcc, SW1 and SW2, CR, CS, CF, and G2 are respectively a supply voltage,selection switches, a reference capacitor 200, an active capacitor 100,a feedback capacitor of a working amplifier G1, and a working amplifierin that order. Let's assume that a parasitic capacitance between thelower electrode 30 a and the semiconductor substrate 10 is present atpoint A. The parasitic capacitance and the wiring resistance causes aprimary delay in the switching frequency of the switch SW1. This delayreduces the accuracy of measurement. If the parasitic capacitance has avoltage dependence, the operation becomes more unstable and the accuracyof measurement becomes worse. If the parasitic capacitance is at pointB, the S/N ratio of the active capacitor (the quantity of capacitancechange and the whole capacitance) becomes greater and the accuracy ofmeasurement becomes worse. Further the parasitic capacitance has avoltage dependence, the output VO becomes unstable.

The reference capacitor 200 in FIG. 1 is a parallel-plate capacitorwhose capacitance is dependent upon electrode area, electrode-electrodedistance and specific inductive capacitance of inter-electrode material.This first embodiment determines the distance between electrodes and theinter-electrode material by the barrier dielectric layer 40. Thespecific inductive capacitance of the barrier dielectric layer 40 usinga CVD nitride layer is approximately 7 to 9. Therefore, the capacitanceapproximately equal to the active capacitor 100 can be attained by aless electrode area. This makes the pressure sensor more inexpensive.

FIG. 3 shows a cross-sectional view of a capacitor type pressure sensorwhich is a second embodiment of the present invention in which thereference capacitor 200 does not contain the barrier dielectric layer40. The reference capacitor 200 of this embodiment comprises a lowerelectrode 30 a, a reference capacitance dielectric 201, an oxide layer202, and an upper electrode 60 a. This configuration can determine thethickness and material of the reference capacitance dielectric 201 whichis an inter-electrode dielectric of the reference capacitor 200separately from the barrier dielectric layer 40. Therefore, thisembodiment can provide inexpensive pressure sensors having lessreference capacitor 200 areas.

FIG. 4 shows a cross-sectional view of a capacitor type pressure sensorwhich is a third embodiment of the present invention. FIG. 5 shows itsplan view.

This embodiment produces the reference capacitor 200 in the same methodas the active capacitor. The reference capacitor 200 comprises areference capacitor stationary electrode 30 d, a cavity 210, and adiaphragm structure 220 and is formed on the semiconductor substrate 10with an oxide dielectric 20 therebetween. The diaphragm structure 220 isconstructed with a stationary scaffold 220 b and a diaphragm electrode220 a that is opposite to the stationary electrode 30 d. The parasiticcapacitance between the semiconductor substrate 10 and the referencecapacitor 200 is small and does not have a voltage dependencesubstantially. Therefore this embodiment can provide a high accuracypressure sensor. The scaffolds 220 b of the reference capacitor 200 arespaced at shorter distances than the scaffolds 120 b of the activecapacitor 100 so that the capacitance of the reference capacitor 200 maynot vary substantially as the surrounding pressure varies. For example,when the scaffolds 220 b spaced at a distance one fourth of the distanceof the scaffolds 120 b, the ratio of a capacitance change of thereference capacitor 200 to the capacitance change of the activecapacitor 100 is about 1/256 because the displacement of the diaphragmstructure 220 is proportional to the fourth power of the space of thestationary scaffolds 220 b.

This configuration enables simultaneous formation of the referencecapacitor stationary electrode 30 d and the active capacitor stationaryelectrode 30 b, the cavity 210 and the cavity 110, and the diaphragmstructure 220 and the diaphragm structure 120. This can also eliminateuneven production of the active capacitor 100 together with thereference capacitor 200. Further this can eliminate characteristicchanges due to external disturbances such as noises of an identicalmember.

FIG. 6 shows a cross-sectional view of a capacitor type pressure sensorwhich is a fourth embodiment of the present invention. FIG. 7 shows itsplan view.

This embodiment has the reference capacitor stationary electrode 30 dformed in the diaphragm structure 120. The reference capacitorstationary electrode 30 d is placed near the scaffold 10 b and theactive capacitor stationary electrode 30 b is placed in the center sothat the displacement of the diaphragm structure 120 may be greatest inthe center of the structure and smallest in the periphery of thestationary scaffolds 120 b. Therefore, the capacitance change of thereference capacitor 200 due to a pressure is not dependent upon thepressure substantially. Although a capacitor type pressure detectingsection having such a configuration to minimize areas has been wellknown, the parasitic capacitance between the semiconductor substrate 10and the reference capacitor stationary electrode 30 d will reduce theaccuracy of measurement when the detecting section and the detectingcircuit 300 are integrated on the semiconductor substrate 10. Therefore,the present invention has solved the problem by forming the referencecapacitor stationary electrode 30 d with an oxide dielectric 20therebetween.

In summary, the present invention can provide a high-accuracy andhigh-stability capacitor type pressure sensor having a very smallparasitic capacitance between the reference capacitor and thesemiconductor substrate and a voltage dependence which can be ignoredsubstantially.

Further, the present invention can integrate the circuit section and thepressure detecting section in a single chip and thus provide a small andinexpensive pressure sensor.

INDUSTRIAL AVAILABILITY OF THE INVENTION

The present invention can provide a high-accuracy and high-stabilitycapacitor type pressure sensor having a very small parasitic capacitancebetween the reference capacitor and the semiconductor substrate and avoltage dependence which can be ignored substantially. Further, thepresent invention can integrate the circuit section and the pressuredetecting section in a single chip and thus provide a small andinexpensive pressure sensor. Furthermore, the present invention canprovide pressure sensors which have preferable characteristics and highreliability also for automobiles.

What we claim is:
 1. A capacitor type pressure sensor comprising anactive capacitor between electrodes whose capacitance varies as thesurrounding pressure varies, a reference capacitor between electrodeswhose capacitance will not vary substantially as the surroundingpressure varies, and a circuit which is electrically connected to bothsaid active and reference capacitors, detects the difference or ratiothereof, and uses the potential of a semiconductor substrate, whereinsaid reference capacitor is configured to act as a conductive electrodeand is provided on the semiconductor substrate through an interveningdielectric substance.
 2. A capacitor type pressure sensor in accordancewith claim 1, wherein said active capacitor is formed on saidsemiconductor substrate with said dielectric therebetween.
 3. Acapacitor type pressure sensor in accordance with claim 1, wherein saidreference capacitor contains a dielectric between electrodes.
 4. Acapacitor type pressure sensor in accordance with claim 1, wherein saidreference capacitor has a cavity between the electrodes and theelectrodes and the cavity of said reference capacitor are simultaneouslyformed with the same material as the active capacitor.
 5. A capacitortype pressure sensor in accordance with claim 4, wherein one electrodeof said reference capacitor has a diaphragm structure of the activecapacitor, said diaphragm structure being secured by scaffolds to thesemiconductor substrate, and the other electrode of said referencecapacitor is placed near the scaffolds in an area delineated by thescaffolds.